The present invention generally relates to the field of communications systems and, more particularly, to the field of radio communications systems wherein transmissions can occur at variable rates.
Commercial communication systems and, in particular, cellular radiotelephone systems have experienced explosive growth in the past decade. This growth is due, at least in part, to the improvement in the number and quality of services provided by radiocommunication systems. For example, early systems were designed primarily to support voice communications. However, cellular radiocommunication systems now provide many additional services including, for example, paging, messaging and data communications (e.g., to support Internet communication). Some of these new services make higher throughputs (i.e., than needed for voice communication) very desirable.
In order to provide these various communication services, a corresponding minimum user bit rate is required. For example, for voice and/or data services, user bit rate corresponds to voice quality and/or data throughput, with a higher user bit rate producing better voice quality and/or higher data throughput. The total user bit rate is determined by a selected combination of techniques, e.g., speech coding, channel coding, modulation scheme, and the air interface resources allocated to the connection, i.e., for a TDMA system, the number of assignable time slots, for a CDMA system the number of spreading codes.
Today's cellular phones-transmit at a net data rate of about 10 kbit/s. In the future, it is expected that cellular modems will be able to receive and transmit several hundreds of kilobits per second. One example is the GSM-based packet data system referred to as General Packet Radio Service (GPRS). However, in order to provide these higher throughput rates while at the same time maintaining existing cell sizes (which latter criteria is strongly desired by network operators), the transmit power must increase correspondingly. Under this scenario, especially for small wireless modems, such as those which can be built-in to small handheld phones and for wireless modem cards inserted into PCs or laptops, an increase in average power will generate more heat than can be cooled off by these small devices.
For example, power amplifiers used in mobile phones and modems in the transmit path are not perfect, i.e., not all power is transformed into electromechanical waves. Depending on the modulation scheme and implementation, roughly half of the generated power is lost in the form of heat dissipation in the power amplifier. This heat can be damaging to the modem or annoying to the end user in the case of a handheld device. Thus, lack of heat dissipation rather than issues of complexity associated with higher bit rates may limit the maximum bit rate that a small phone can transmit.
For packet data operation the activity is often very bursty. This reduces the average power consumption. The burstyness of packet data transmissions is governed by the application, i.e., the instantaneous use. However, the mobile phone designer typically assumes the worst case scenario, i.e., that there will be times when the application will transmit for an extended period of time (related to the heat dissipation time constant) at its full bit rate capability. Thus, mobiles may be designed to restrict their transmitted bit rate such that the temperature of the devices is limited to a safe level even during worst case periods of usage.
Some documents describe attempts to combat increases in device temperature by monitoring the device temperature and adjusting the operation of the device based on the monitored temperature. For example, EP 800,282 describes a system wherein a temperature sensor monitors the temperature within a portion of the system. When the monitored temperature exceeds a threshold temperature, then the transmission rate associated with a speech codec is decreased. Similarly, the abstract of JP 9/326749 describes a system wherein data packets are transmitted in consecutive timeslots when a temperature of a power module is less than a threshold, but wherein packet data transmission is made intermittently when the power module becomes too hot.
Although these conventional solutions partially address the temperature/transmission rate problem described above, they do so in a unilateral manner, i.e, the mobile station makes a temperature determination and unilaterally adjusts transmissions accordingly. Applicant envisions additional features which provide a cooperative solution between the system, mobile unit and user for addressing temperature/transmission rate issues, whereby overall system operation and user knowledge is enhanced.